It is known to change the gear ratio of a gear drive, i.e. the ratio between the number of revolutions of two connected shafts of the gear drive, by positioning multiple gears on a single splined shaft to slide into engagement or disengagement with other gears mounted on one or more parallel adjacent shafts. Such a gear drive is used in automotive transmissions, for example.
In an alternative design, providing a more compact drive and a simpler shifting procedure, a number of idler shafts and idler gears are simultaneously driven at different gear ratios by a single drive shaft. An output shaft is moved in a circular orbit to engage selected ones of the idler shafts.
This latter design suffers from the drawback that all the idlers are constantly in motion and the input and output shafts are both unaligned.